Many devices, such as tuners, receivers, transmitters, upconverters, downconverters, and so forth, require the generation of oscillator signals having precise frequencies. Synthesizers (which typically include voltage controlled oscillators (VCOs)) produce output signals at desired frequencies. For instance, a fractional-N synthesizer generates an output signal having a frequency that is expressed as an integer number of a reference frequency plus a fractional amount of the reference frequency. The reference frequency is typically provided by a crystal oscillator. As such, fractional N synthesizers are often employed for their ability to produce a very fine frequency step, as well as the ability to achieve low phase noise performance.
However, in certain situations (such as when a VCO, a crystal oscillator reference, and other logic are implemented on a single die) unwanted interactions can cause spurs at an offset frequency from the desired frequency. This offset frequency is typically the difference between the VCO frequency and the nearest harmonic of the crystal oscillator reference frequency.
These spurs are not a problem with an integer-N synthesizer, as they would be aligned with the desired frequency. However, in a fractional-N synthesizer, the VCO frequency can be at a small offset from a harmonic of the crystal oscillator frequency.
Typically, the level of such spurs falls off at 6 dB per octave with increasing offset frequency. Thus, in a typical system, such spurs will be sufficiently attenuated once the offset exceeds a particular threshold (e.g., 50 kHz). However, if the required synthesized frequency is close to a harmonic of the crystal oscillator frequency, then spurs may present problems such as interference and degraded performance.